import_realm: Improve comment about sequencing of imported models.

The comment was outdated, currently we import UserProfiles before
realm_tables - because some models in realm_tables have a dependency on
UserProfile.

Also makes sense to elaborate a bit more in the comment that it's just
an outline of the ordering, not an exhaustive list.

zerver/lib/import_realm.py

@@ -902,13 +902,18 @@ def import_uploads(
 # Importing data suffers from a difficult ordering problem because of
 # models that reference each other circularly.  Here is a correct order.
 #
+# (Note that this list is not exhaustive and only talks about the main,
+# most important models. There's a bunch of minor models that are handled
+# separately and not mentioned here - but following the principle that we
+# have to import the dependencies first.)
+#
 # * Client [no deps]
 # * Realm [-notifications_stream]
 # * UserGroup
 # * Stream [only depends on realm]
 # * Realm's notifications_stream
-# * Now can do all realm_tables
 # * UserProfile, in order by ID to avoid bot loop issues
+# * Now can do all realm_tables
 # * Huddle
 # * Recipient
 # * Subscription